Internal combustion engines generate particle emissions that are undesirable for a variety of reasons. It is well known that waste products in engine exhaust such as carbon monoxide, hydrocarbons, and nitrogen oxides adversely affect human health, and present risks to the environment. Diesel engines in particular produce considerable amounts of soot, which contains particulate matter, black carbon, sulfur dioxide, nitrogen oxides and other hazardous pollutants. Several government agencies regulate emissions of such material.
To meet these regulations, conventional diesel exhaust systems include catalyst soot filters to collect the soot resulting from fuel combustion, and reduce the amount of soot that exits the tailpipe. Such soot filters typically include aluminum-oxide coated meshes, ceramic fibers, ceramic honeycomb structures or ceramic foams. Over time, soot filters become clogged, and must be regenerated. Generally, the regeneration process includes burning the accumulated soot to convert it into less harmful material. One method for accomplishing periodic regeneration is to allow the soot filter to become clogged with soot and accumulate heat due to increased engine speed or load. Eventually, the heat increases to the temperature required for ignition of the carbon constituent of the soot (typically 550 to 600 degrees C.). Under certain load conditions, however, the exhaust gas temperatures do not reach these levels.